Capacitors store electrical charge. They are made up of two conducting plates separated by a dielectric (insulator) or gap. A positive and a negative charge builds up on the opposite plates, but the insulator stops the charge from moving – this creates a potential difference via a uniform electric field. The charges on each plate are equal and opposite.
Capacitance is the of a capacitor is defined as the charge stored per unit p.d. across the capacitor.
C =Q/V capacitance = charge / potential difference
The units of capacitance are Farads, F, or Coulomb per Volt, C/V.
As a general rule of thumb, everything about capacitors is the opposite to how it would be with resistors:
1/C = 1/C(1) + 1/C(2) … Combined Capacitance in series
C = C(1) + C(2) … Combined Capacitance in parallel
Work is done removing negative charge from one plate and depositing it on the other plate – this is the same as the energy stored by the capacitor, and can be found as the area under a p.d. – charge graph:
W = 1/2 QV Energy stored = 1/2 x charge x p.d.
This equation can be rewritten using the capacitance equation, C = Q/V, rearranged as Q = CV
W = 1/2 V²C Energy Stored = 1/2 voltage² x capacitance
Or, using the capacitance equation written in the form V = Q/C: